The use of piezoelectric crystals, such as quartz, as frequency standards constitutes an important application of piezoelectricity technically as well as commercially. A variety of piezoelectric crystal devices, such as crystal-based oscillator circuits, are commercially available and their use in the generation of precise frequency control and timing is extremely common and relies on the normally high quality factor of such piezoelectric crystals as mechanical resonators. Quartz crystals, for instance, serve as good frequency standards by virtue of their extremely high internal quality factor (i.e., the quality factor due to internal frictional losses, excluding mounting losses and air losses) which can attain a value of over 10.sup.7 at a frequency of around 1 MHz. In addition, quartz exhibits high quality as a dielectric along with a low dielectric constant and is relatively easy to be cut and polished.
Piezoelectric crystal oscillators are capable of generating a wide range of precise frequencies at a given temperature, but for a given oscillator substantial variations in the frequency of operation occur as the temperature to which the crystal is subjected varies over a relatively large range. Although the frequency versus temperature characteristics of piezoelectric crystals are determined primarily by the angles of cut of the crystal plates with respect to the crystallographic axes of quartz, and the points of zero temperature coefficient can be varied over a wide range by varying the angles of cut, the frequency of oscillation for any given angle of cut remains substantially temperature dependent.
To achieve higher stability of oscillating frequencies, oven-controlled oscillators, in which the crystal unit and the temperature-sensitive components of the oscillator are placed in a stable oven whose temperature is set to the crystal's turnover temperature, are used. Such oven-controlled oscillators are bulky and consume added power. In addition, an oven-controlled oscillator requires about 10 minutes after being turned on in order to stabilize. Significant frequency shifts are also produced because of thermal stresses in the crystal during warm up and this thermal-transient effect can make the typical warm up time of a crystal oscillator longer than the time it takes for the oven to stabilize.
Problems associated with a drift in oscillator frequency as the temperature of operation fluctuates substantially are also prevalent in other kinds of voltage-controlled oscillators and limits their use in frequency sensitive applications.
Some of the above problems have been solved, at least in part, by temperature-compensated crystal oscillators (TCXO's) which use to advantage the fact that in crystal oscillators, the crystal unit is offered a load capacity by the oscillator circuit and the oscillator operates at the frequency where the crystal unit's reactance cancels the reactance of a load capacitor. The oscillator frequency is dependent upon the load capacitance of the oscillator circuit, and TCXO's function by compensating the frequency versus temperature behavior of the crystal by varying the load capacitance appropriately. Generally, the output signal from a temperature sensor in the form of a thermistor network is used to generate the correction voltage applied to a varactor in order to maintain frequency stability. Such TCXO's are capable of providing high frequency stability over a reasonable temperature range, are smaller and consume less power than oven-controlled oscillators, and require no lengthy warm up time. Conventional TCXO's generally use some sort of trimmed nonlinear analogue networks which generate control voltages for the crystal oscillator on the basis of input from a temperature sensor, and require complex measurement and trim algorithms in the presence of multiple nonlinearities and scale factors. In addition, analog trimming has a limited range and entails the use of expensive and cumbersome hardware.
A further factor affecting the stability of the frequency of a crystal unit is the gradual change brought about as a result of crystal aging. Aging can result from a variety of causes such as mass transfer to or from the resonator's surfaces due to adsorption and desorption of contamination, and stress relief within the mounting structure or at the interface between the quartz crystal and the electrodes contacting the crystal. Although crystal aging observed at constant temperature usually follows an approximately logarithmic dependence on time and aging rate decreases as time elapses so that stabilization occurs within the unit each time the temperature of a crystal changes substantially, a new aging cycle starts and is accompanied by a corresponding frequency instability.
Crystals capable of increased frequency stability in the presence of temperature fluctuation and aging effects are available for frequency-sensitive operations but involve considerably increased cost due to the need for clean glass, metal or ceramic enclosures and advanced surface cleaning, packaging and ultra high vacuum fabrication techniques. The increased cost and accompanying bulk of such crystals constitutes a severe limitation to their use in most commercial applications requiring high frequency stability. Some currently available TCXO's do provide a relatively precise output frequency (variation of about .+-.5 ppm) over a limited temperature range, but these TCXO's use high-precision crystals which are costly because of the highly precise angle of cut and elaborate packaging.
The use of crystal oscillators as reference modules in cellular telephones in particular requires them to be extremely compact in order to keep the size of the phones as small as possible. In addition, this application requires a high degree of frequency stability in order to keep the operation of the cellular phones in conformance with strict FCC requirements. As a result, conventional crystal oscillator-based cellular phones have to be serviced at least once a year to adjust the oscillator unit to account for any frequency instability due to crystal aging.
Hence, there exists a need for a temperature-compensated oscillator which is capable of producing a stable output frequency over a wide range of temperature, which can easily be compensated for the effects of frequency drifts due to other factors, such as crystal aging in TCXO's, and which can be fabricated in the form of an economical and compact unit.